African Horse Sickness Viruses

African horse sickness is a highly fatal, noncontagious disease of equines, particularly horses. The African horse sickness virus group consists of nine serotypes, in the genus Orbivirus, family Reoviridae. It is closely related to the bluetongue viruses, which cause disease in cattle and sheep. Infection with any of the serotypes of African horse sickness virus usually results in severe disease and high mortality in horses. Donkeys and mules generally exhibit less severe disease and lower mortality, while wild equids such as zebra generally show no signs of disease or mortality aſter infection. The serotypes are differentiated based on the host immune response, and there is some cross reaction between serotypes.

All nine serotypes are endemic to sub-Saharan Africa, and have caused serious epidemics when introduced outside this area. African horse sickness has had a significant impact on the history of some parts of Africa, as horses could not be used in exploration and farming. Outbreaks of African horse sickness have a significant economic impact, resulting from the direct loss of animals, the costs of control programs, and trade regulations and quarantines restricting movement of equines from infected areas.

The virus is transmitted by biting midges in the genus Culicoides. Culicoides imicola has been implicated in most outbreaks, while in endemic areas there may be several species of Culicoides midge involved in the transmission cycle. Most species of equines can develop viremias sufficiently high to infect midges. Some tick species have been shown to be able to become infected and transmit the virus in the laboratory, but the importance of this transmission route in nature is unknown.

In endemic areas, African horse sickness viruses circulate primarily between midges and zebra, and frequently multiple serotypes are present. Transmission rates can be very high. For example, in the Kruger National Park, South Africa, zebra foals typically are exposed to all nine serotypes by the time they are 1 year old.

The first outbreak outside the sub-Saharan zone began in 1959 in Saudi Arabia and Iran, spreading to involve Afghanistan, Pakistan, Syria, Lebanon, Jordan, Iraq, Turkey, Cyprus and parts of India before being controlled by vaccination campaigns and the loss of most susceptible horses by the end of 1961. Another outbreak occurred in North Africa in 1965, crossing into Spain in 1966. An outbreak of African horse sickness serotype 4 virus began in central Spain in 1987 and ultimately encompassed a large part of Spain, along with parts of Portugal and Morocco. This outbreak was the first recorded instance of an African horse sickness virus overwintering outside of Africa. African horse sickness cases occurred for four subsequent years in Spain, and it was not eradicated until 1990. The most likely route of introduction was via zebra imported from Namibia. Control and eradication of the virus was achieved only by extensive vaccination campaigns and slaughter of infected or exposed equines. It is estimated that 2,000 horses died and over 350,000 were vaccinated during this outbreak. In 1989, an outbreak of serotype 9 occurred in Saudi Arabia.

Spain was divided into African horse sickness-free and infected regions, in order to allow movement of horses for the 1992 Olympics held in Barcelona. No vaccination was allowed in the African horse sickness-free region, so that any transmission activity would be observed. Equine movement out of the infected region was prohibited.

A similar strategy is used in South Africa, with an African horse sickness-free zone in the Western Cape Province, based on historically low incidence of African horse sickness, and that C. imicola is rare. Surrounding this zone is a surveillance zone and a protection zone. No vaccination is allowed in the free and surveillance zones, and strict movement controls are in place for equines moving from other areas of the country. The zoning creates an area where animals can be held prior to exportation. In 1999, there was an outbreak of African horse sickness in the surveillance zone, opening debate about the effectiveness of the movement restrictions and the vector species involved. Currently, the zoning is still in place and there have not been further outbreaks in the African horse sickness-free zone.

Clinical signs of African horse sickness in horses generally begin with fever. There are three forms of African horse sickness disease in horses: pulmonary, cardiac, and febrile. The febrile form, often referred to as horse sickness fever, does not progress beyond fever and generally resolves. The pulmonary form begins with fever and progresses to respiratory difficulty, coughing and nasal discharge. Death is due to pulmonary edema and cardiac failure. The cardiac form also begins with fever, and subsequently edemas develop around the head, neck and chest. Death results from cardiac insufficiency and progressive pulmonary edema. Mortality rates can be as high as 95% in horses, although donkeys and mules are much less susceptible and mortality rates are much lower. Movement of donkeys may be important in the spread of these viruses outside the endemic area, as they are rarely clinically ill and so infected animals are not noticed. There is variation in virulence between the nine African horse sickness serotypes, along with differences between breeds of horse and individual immune responses to the virus.

Vaccines have been developed against these viruses, but due to the immunological differentiation between the serotypes, cross protection is not complete and full protection requires vaccination against each serotype. Routine vaccination of horses, against all serotypes, is practiced in endemic areas. Elsewhere, vaccination generally is not used routinely or is not allowed. In outbreak situations, the virus is first typed to determine the serotype involved, then vaccination is targeted against that serotype only.

Most countries restrict importation of equids from endemic countries. An extended quarantine period usually is imposed, thus restricting the movement of horses for competition. Because it is difficult to differentiate between vaccinated and infected animals, generally there are restrictions on importing vaccinated animals.

Antibodies to African horse sickness viruses have been found in other animals such as elephants, camels, and bovines, but there is no apparent illness and their impact on the transmission cycle is not known. Dogs can become infected and die by eating meat from the carcass of an infected animal. Antibodies to African horse sickness have been found in wild canids and other carnivores, most likely also via feeding on infected carcasses. Humans have been infected only rarely, generally through laboratory accidents with vaccine strains.

African horse sickness has never been found in the New World. However, there are species of Culicoides, particularly the C. variipennis complex, present throughout the U.S. Some members of this complex are competent vectors of African horse sickness viruses in the laboratory, and will be competent vectors in the field should an African horse sickness virus be introduced. Vector competence for the C. variipennis complex varies considerably for the closely related bluetongue viruses, but we lack information on similar variation for African horse sickness viruses. The implications of this for an introduction of any of the African horse sickness viruses is unknown, but requires further study.